Fuel compositions

ABSTRACT

A fuel composition comprising a base fuel and at least one viscosity index (VI) improving additive, wherein the viscosity index (VI) improving additive is a star-shaped polyisoprene-based polymer. The viscosity index improving additive can be used in a fuel composition to provide improved lubricity as well as providing improved power output and/or acceleration characteristics.

FIELD OF THE INVENTION

The present invention relates to automotive fuel compositions, and in particular to automotive fuel compositions comprising viscosity index improver (VII) components.

BACKGROUND OF THE INVENTION

Viscosity index (VI) is a commonly used method of measuring a fluid's change of viscosity in relation to temperature. The higher the VI, the smaller the relative change in viscosity with temperature. VI improvers (also known as viscosity modifiers) are additives that increase the viscosity of the fluid throughout its useful temperature range.

It is known to use a viscosity increasing component in a fuel composition in order to improve acceleration performance. WO2009/118302 describes the use of a viscosity index (VI) improving additive, in an automotive fuel composition, for the purpose of improving the acceleration performance of an internal combustion engine into which the fuel composition is or is intended to be introduced or of a vehicle powered by such an engine.

It would be desirable to be able to further improve the performance of a vehicle engine, by altering the composition and/or properties of the fuel introduced into it, as this can be expected to provide a more simple, flexible and cost effective route to performance optimisation than by making structural or operational changes to the engine itself.

In particular, as well as identifying new viscosity index (VI) improving additives which provide improvements in power and/or acceleration properties, it would be desirable to improve one or more other aspects of the fuel composition such as, for example, friction modification properties, lubricity, engine cleanliness, cold temperature performance and pumpability.

US2013/0165362 discloses certain linear and star polymers suitable for use as a viscosity index improver for lubricating oil compositions. However there is no disclosure in US2013/0165362 of the use of such polymers in a fuel composition.

SUMMARY OF THE INVENTION

According to the present invention there is provided a fuel composition comprising a base fuel and a viscosity index improver, wherein the viscosity index improver comprises a star-shaped polyisoprene-based polymer.

According to a second aspect of the present invention there is provided the use of a viscosity index improver in a fuel composition for providing improved lubricity, wherein the viscosity index improver comprises a star-shaped polyisoprene-based polymer.

According to a further aspect of the present invention there is provided the use of a viscosity index improver in a fuel composition for providing improved power output and/or acceleration characteristics, wherein the viscosity index improver is a star-shaped polyisoprene-based polymer.

It has surprisingly been found that the viscosity index improvers described herein can be used in a fuel composition to provide improved lubricity as well as providing improved power output and/or acceleration characteristics.

Hence, according to yet a further aspect of the present invention there is provided the use of a viscosity index (VI) improving additive in a fuel composition for the purpose of improving the lubricity of the fuel composition at the same time as improving the power output and/or acceleration performance of an internal combustion engine into which the viscosity index (VI) improving additive, or the fuel composition containing the viscosity index (VI) improving additive, is or is intended to be introduced or of a vehicle powered by such an engine, wherein the viscosity index (VI) improving additive is a polyisoprene-based star polymer.

It has also been found that the viscosity index improvers used in the fuel compositions herein can provide one or more of improved friction modification properties, improved filterability properties, improved viscosity properties, improved low temperature performance, improved pumpability especially at low temperatures and no increase in engine fouling.

DETAILED DESCRIPTION OF THE INVENTION

As used herein the term ‘viscosity index (VI) improver’ means an additive that increases the viscosity of the fuel throughout its useful temperature range. Viscosity index improvers are also known as viscosity modifiers.

The fuel composition described herein is preferably a diesel fuel composition and the internal combustion engine described herein is preferably a diesel engine.

By “diesel engine” is meant a compression ignition internal combustion engine, which is adapted to run on a diesel fuel.

In addition to using the present invention for improving the power output and/or acceleration characteristics of the engine, the present invention can be of use in improving the lubricity of a fuel composition. As used herein, the term “lubricity” in relation to a fuel means the capacity of the fuel for reducing friction and/or wear in an internal combustion engine.

Lubricity performance may be assessed by measuring the engine wear. Engine wear may be measured by any suitable method. A suitable method for measuring the engine wear is the HFRR (High-Friction Reciprocating Rig) test ISO 12156. In the context of the present invention, an “improvement” in lubricity performance embraces any degree of improvement. Similarly a reduction or increase in a measured parameter—for example the reduction in engine wear provided by a fuel composition—embraces any degree of reduction or increase, as the case may be. The improvement, reduction or increase—as the case may be—may be as compared to the relevant parameter when using the fuel composition prior to incorporation of the viscosity index (VI) improving additive. It may be as compared to the relevant parameter measured when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in an internal combustion (typically diesel) engine, prior to adding a viscosity index (VI) improving additive to it.

The present invention may, for example, involve adjusting the properties and/or performance and/or effects of the fuel composition, in particular its effect on the lubricity performance of a fuel composition, by means of the viscosity index (VI) improving additive, in order to meet a desired target.

An improvement in lubricity performance may also embrace mitigation, to at least a degree, of a decrease in lubricity performance due to another cause, in particular due to another fuel component or additive included in the fuel composition.

An improvement in lubricity performance may also embrace restoration, at least partially, of lubricity performance which has been reduced for another reason such as desulphurisation, hydrotreatment or hydrocracking of diesel and diesel components.

“Acceleration performance” includes generally the responsiveness of the engine to increased throttle, for example the rate at which it accelerates from any given engine speed. It includes the level of power and/or torque and/or vehicle tractive effort (VTE) generated by the engine at any given speed. Thus an improvement in acceleration performance may be manifested by an increase in engine power and/or torque and/or VTE at any given speed.

Engine torque may be derived from the force exerted on a dynamometer by the wheel(s) of a vehicle which is powered by the engine under test. It may, using suitably specialised equipment (for example the Kistler™ RoaDyn™), be measured directly from the wheels of such a vehicle. Engine power may suitably be derived from measured engine torque and engine speed values, as is known in the art. VTE may be measured by measuring the force exerted, for example on the roller of a chassis dynamometer, by the wheels of a vehicle driven by the engine.

The present invention can be of use in improving the acceleration performance of an internal combustion engine or of a vehicle powered by such an engine. Acceleration performance may be assessed by accelerating the engine and monitoring changes in engine speed, power, torque and/or VTE, air charge pressure and/or turbo charger speed with time. This assessment may suitably be carried out over a range of engine speeds.

Acceleration performance may also be assessed by a suitably experienced driver accelerating a vehicle which is powered by the engine under test, for instance from 0 to 100 km/hour, on a road. The vehicle should be equipped with appropriate instrumentation such as an engine speedometer, to enable changes in acceleration performance to be related to engine speed.

In general, an improvement in acceleration performance may be manifested by reduced acceleration times, and/or by any one or more of the effects described above, for example a faster increase in turbo charger speed, or an increase in engine torque or power or VTE at any given speed.

In the context of the present invention, an “improvement” in acceleration performance embraces any degree of improvement. Similarly a reduction or increase in a measured parameter—for example the time taken for the turbo charger to reach its maximum speed—embraces any degree of reduction or increase, as the case may be. The improvement, reduction or increase—as the case may be—may be as compared to the relevant parameter when using the fuel composition prior to incorporation of the viscosity index (VI) improving additive. It may be as compared to the relevant parameter measured when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in an internal combustion (typically diesel) engine, prior to adding a viscosity index (VI) improving additive to it.

The present invention may, for example, involve adjusting the properties and/or performance and/or effects of the fuel composition, in particular its effect on the acceleration and/or power output performance of an internal combustion engine, by means of the viscosity index (VI) improver, in order to meet a desired target.

An improvement in acceleration performance may also embrace mitigation, to at least a degree, of a decrease in acceleration performance due to another cause, in particular due to another fuel component or additive included in the fuel composition. By way of example, a fuel composition may contain one or more components intended to reduce its overall density so as to reduce the level of emissions which it generates on combustion; a reduction in density can result in loss of engine power, but this effect may be overcome or at least mitigated by the use of a viscosity index (VI) improver in accordance with the present invention.

An improvement in acceleration performance may also embrace restoration, at least partially, of acceleration performance which has been reduced for another reason such as the use of a fuel containing an oxygenated component (e.g. a so-called “biofuel”), or the build-up of combustion related deposits in the engine (typically in the fuel injectors).

Where the present invention is used to increase the engine torque, typically during a period of acceleration, at a given engine speed, the increase may be of at least 0.1%, preferably of at least 0.2 or 0.3 or 0.4 or 0.5%, in cases of at least 0.6 or 0.7%, compared to that obtained when running the engine on the fuel composition prior to incorporation of the viscosity index (VI) improver. The increase may be as compared to the engine torque obtained at the relevant speed when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in an internal combustion (typically diesel) engine prior to adding a viscosity index (VI) improver to it.

Where the present invention is used to increase the engine power, typically during a period of acceleration, at a given engine speed, the increase may again be of at least 0.1%, preferably of at least 0.2 or 0.3 or 0.4 or 0.5%, in cases of at least 0.6 or 0.7%, compared to that obtained when running the engine on the fuel composition prior to incorporation of the viscosity index improver. The increase may be as compared to the engine power obtained at the relevant speed when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in an internal combustion (typically diesel) engine prior to adding a viscosity index improver to it.

Where the present invention is used to increase the engine VTE, typically during a period of acceleration, at a given engine speed, the increase may again be of at least 0.1%, preferably of at least 0.2 or 0.3 or 0.4 or 0.5%, in cases of at least 0.6 or 0.7%, compared to that obtained when running the engine on the fuel composition prior to incorporation of the viscosity index (VI) improver. The increase may be as compared to the VTE obtained at the relevant speed when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in an internal combustion (typically diesel) engine prior to adding a viscosity index (VI) improver to it.

Where the present invention is used to reduce the time taken for the engine to accelerate between two given engine speeds, the reduction may be of at least 0.1%, preferably of at least 0.2 or 0.3 or 0.4 or 0.5%, in cases of at least 0.6 or 0.7 or 0.8 or 0.9%, compared to that taken when running the engine on the fuel composition prior to incorporation of the viscosity index (VI) improver. The reduction may be as compared to the acceleration time between the relevant speeds when the same engine is run on an otherwise analogous fuel composition which is intended (e.g. marketed) for use in an internal combustion (typically diesel) engine prior to adding a viscosity index (VI) improver to it. Such acceleration times may for instance be measured over an engine speed increase of 300 rpm or more, or of 400 or 500 or 600 or 700 or 800 or 900 or 1000 rpm or more, for example from 1300 to 1600 rpm, or from 1600 to 2200 rpm, or from 2200 to 3000 rpm, or from 3000 to 4000 rpm.

The automotive fuel composition in which the VI improving additive is used, in accordance with the present invention, may in particular be a diesel fuel composition suitable for use in a diesel engine. It may be used in, and/or may be suitable and/or adapted and/or intended for use in, any type of compression ignition engine, for instance those described below.

Viscosity index improving additives (also referred to as VI improvers) are already well known for use in lubricant formulations, where they are used to maintain viscosity as constant as possible over a desired temperature range by increasing viscosity at higher temperatures. They are typically based on relatively high molecular weight, long chain polymeric molecules that can form conglomerates and/or micelles. These molecular systems expand at higher temperatures, thus further restricting their movement relative to one another and in turn increasing the viscosity of the system.

The VI improving additives used in a fuel composition in accordance with the present invention comprise polyisoprene-based star polymers, especially styrene-isoprene star polymers.

Examples of suitable styrene-isoprene star polymers include those disclosed in US2013/0165362, incorporated herein by reference in its entirety. The star polymers disclosed in US2013/0165362 have multiple triblock arms coupled to a central core, such as a divinylbenzene (DVB) core, wherein the triblock arms contain a block derived from monoalkenyl arene monomer positioned between two partially or fully hydrogenated blocks derived from diene, wherein at least one of the diene blocks is a copolymer derived from mixed diene monomer, in which from about 65 wt % to about 95 wt % of the incorporated monomer units are from isoprene and from about 5 wt % up to about 35 wt % of the incorporated monomer units are from butadiene, and wherein at least about 80 wt %, preferably at least about 90 wt % butadiene is incorporated into the random copolymer block in a 1,4-configuration.

Preferred viscosity index (VI) improver additives for use in the fuel compositions are the star-shaped isoprene polymers described in US2013/0165362 which can be characterized by the formula:

(D′-PA-D″)n-X;

wherein D′ represents an “outer” block derived from diene; PA represents a block derived from monoalkenyl arene; D″ represents an inner random derived from diene; n represents the average number of arms per star polymer formed by the reaction of 2 or more moles of a polyalkenyl coupling agent per mole of arms and X represents a nucleus of a polyalkenyl coupling agent.

At least one of diene blocks D′ and D″, preferably each of diene blocks D′ and D″, are copolymer blocks derived from mixed diene monomer, in which from about 65 wt % to about 95 wt % of the incorporated monomer units are from isoprene and from about 5 wt % up to about 35 wt % of the incorporated monomer units are from butadiene, and wherein at least about 80 wt % of butadiene, preferably at least 90 wt % of the butadiene is incorporated in a 1,4-configuration. Preferably, at least about 15 wt % of the incorporated monomer units are butadiene monomer units. Preferably, no greater than about 28 wt % of the incorporated monomer units are butadiene monomer units. Preferably, at least one of diene blocks D′ and D″, more preferably each of diene blocks D′ and D″, are random copolymer blocks. Blocks D′ and D″ are preferably hydrogenated to remove at least about 80% or 90% or 95% of unsaturations, and more preferably, are fully hydrogenated.

Outer block D′ has a number average molecular weight of from about 10,000 to about 120,000 daltons, more preferably from about 20,000 to about 60,000 daltons, before hydrogenation. Block PA has a number average molecular weight of from about 10,000 to about 50,000 daltons. Increasing the size of block PA can adversely affect the thickening efficiency of the star polymer. Therefore, the number average molecular weight of block PA is preferably from about 12,000 to about 35,000 daltons. Inner block D″ has a number average molecular weight of from about 5,000 to about 60,000 daltons, more preferably from about 10,000 to about 30,000 daltons, before hydrogenation. The term “number average molecular weight”, as used herein, refers to the number average molecular weight as measured by Gel Permeation Chromatography (“GPC”) with a polystyrene standard.

In the star polymers used herein, the ratio of the number average molecular weight of outer block D′ to the number average molecular weight of inner block D″ is preferably at least about 1.4:1, such as at least about 1.9:1, more preferably at least about 2.0:1, and the ratio of the number average molecular weight of block PA to the number average molecular weight of inner block D″ is preferably at least about 0.75:1, such as at least about 0.9:1, more preferably at least about 1.0:1.

Preferably, no greater than 30 wt %, more preferably no greater than 25 wt %, of the total amount of polydiene in the star polymers is derived from butadiene. Preferably, at least about 80 wt %, more preferably at least about 90 wt %, of the total amount of butadiene, which can be incorporated in the polymer as 1,2- or 1,4-configuration units, is incorporated into the star polymer is incorporated in a 1,4-configuration. Increasing the percentage of butadiene incorporated into the polymer as 1,4-units can increase the thickening efficiency properties of the star polymer. An excessive amount of polybutadiene, particularly polybutadiene having a 1,2-configuration, can have an adverse effect on low temperature pumpability properties.

Isoprene monomers used as the precursors of the copolymers herein can be incorporated into the polymer in either a 1,4- or a 3,4-configuration, or a mixture thereof. Preferably, the majority of the isoprene is incorporated into the polymer as 1,4-units, such as greater than about 60 wt %, more preferably greater than about 80 wt %, such as about 80 wt % to 100 wt %, more preferably greater than about 90 wt %, such as about 93 wt % to 100 wt %.

Suitable monoalkenyl arene monomers include monovinyl aromatic compounds, such as styrene, monovinylnaphthalene, as well as the alkylated derivatives thereof, such as o-, m- and p-methylstyrene, alpha-methyl styrene and tertiary butyl styrene. The preferred monoalkenyl arene is styrene.

Star polymers used herein can have from 4 to about 25 arms (n=about 4 to about 25), preferably from about 10 to about 20 arms. Star polymers used herein may have a total number average molecular weight of about 100,000 to about 1,000,000 daltons, preferably from about 400,000 to about 800,000 daltons, more preferably from about 500,000 to about 700,000 daltons.

Further details of the structure, properties and methods of making these preferred star polymers, including various polymerization preparation methods are disclosed in US2013/0165362, incorporated herein by reference in its entirety.

Another suitable type of viscosity index (VI) improving additive for use herein is a star-shaped polyisoprene polymer comprising a crosslinked polystyrene core with arms of hydrogenated polyisoprene or poly(alternated ethylene-propylene). An example of such a polymer is SV300, commercially available from Infineum. As disclosed in US2017/025370, SV300 contains 6% by weight of crosslinked polystyrene star core with 30 arms of hydrogenated polyisoprene, or poly(alternated ethylene-propylene), with overall molecular weight of 875,000 and a hydrodynamic radius in PA04 (a polyalphaolefin having a viscosity at 100° C. of approximately 4 mm²/s commercially available from Exxon Mobil) of 25 nm.

The density of the viscosity index (VI) improving additive for use herein at 15.6° C. (ASTM D-4052) is 0.70 g/cm³ or greater, preferably 0.75 g/cm³ or greater.

Examples of commercially available viscosity index (VI) improving additives suitable for use herein include those commercially available from Infineum under the trade designations SV300, SV600, SV260 and the like.

An especially preferred viscosity index (VI) improving additive from the viewpoint of improving lubricity characteristics of the fuel composition as well as providing improved power output characteristics is SV600, commercially available from Infineum.

The VI improving additive may be pre-dissolved in a suitable solvent, for example an oil such as a mineral oil or Fischer-Tropsch derived hydrocarbon mixture; a fuel component (which again may be either mineral or Fischer-Tropsch derived) compatible with the fuel composition in which the additive is to be used (for example a middle distillate fuel component such as a gas oil or kerosene, when intended for use in a diesel fuel composition); a poly alpha olefin; a so-called biofuel such as a fatty acid alkyl ester (FAAE), a Fischer-Tropsch derived biomass-to-liquid synthesis product, a hydrogenated vegetable oil, a waste or algae oil or an alcohol such as ethanol; an aromatic solvent; any other hydrocarbon or organic solvent; or a mixture thereof. Preferred solvents for use in this context are mineral oil based diesel fuel components and solvents, and Fischer-Tropsch derived components such as the “XtL” components referred to below. Biofuel solvents may also be preferred in certain cases.

The concentration of the VI improving additive in the fuel composition may be up to 1% w/w (10,000 ppm), suitably up to 0.5% w/w, in cases up to 0.4 or 0.3 or 0.25% w/w. It may be 0.001% w/w or greater, preferably 0.01% w/w or greater, suitably 0.02 or 0.03 or 0.04 or 0.05% w/w or greater, in cases 0.1 or 0.2% w/w or greater. Suitable concentrations may for instance be from 0.001 to 1% w/w, or from 0.001 to 0.5% w/w, or from 0.05 to 0.5% w/w, or from 0.05 to 0.25% w/w, for example from 0.05 to 0.25% w/w or from 0.1 to 0.2% w/w.

The remainder of the composition will typically consist of one or more automotive base fuels, for instance as described in more detail below, optionally together with one or more fuel additives.

The above concentrations are for the VI improving additive itself, and do not take account of any solvent(s) with which its active ingredient is pre-diluted. They are based on the mass of the overall fuel composition. Two or more VI improving additives can be used in the fuel composition herein. Where a combination of two or more VI improving additives is used in the composition, the same concentration ranges may apply to the overall combination, again minus any pre-solvent(s) present.

The concentration of the VI improving additive will depend on the desired viscosity of the overall fuel composition, the viscosity of the composition prior to incorporation of the additive, the viscosity of the additive itself and the viscosity of any solvent in which the additive is used. The relative proportions of the VI improving additive, fuel component(s) and any other components or additives present, in an automotive fuel composition prepared according to the present invention, may also depend on other desired properties such as density, emissions performance and cetane number, in particular density.

It has surprisingly been found that the VI improving additive described herein can increase the lubricity of the fuel composition, as well as increasing the power output and/or acceleration characteristics.

Due to the inclusion of the VI improving additive, a fuel composition prepared according to the present invention (in particular a diesel fuel composition) will suitably have a VK 40 of 2.7 or 2.8 mm²/s or greater, preferably 2.9 or 3.0 or 3.1 or 3.2 or 3.3 or 3.4 mm²/s or greater, in cases 3.5 or 3.6 or 3.7 or 3.8 or 3.9 or even 4 mm²/s or greater. Its VK 40 may be up to 4.5 or 4.4 or 4.3 mm²/s. In certain cases, for example arctic diesel fuels, the VK 40 of the composition may be as low as 1.5 mm²/s, although it is preferably 1.7 or 2.0 mm²/s or greater. References in this specification to viscosity are, unless otherwise specified, intended to mean kinematic viscosity.

The composition preferably has a relatively high density, for example for a diesel fuel composition 830 kg/m³ or greater at 15° C. (ASTM D-4052 or EN ISO 3675), preferably 832 kg/m³ or greater, such as from 832 to 860 kg/m³. Suitably its density will be no higher than 845 kg/m³ at 15° C., which is the upper limit of the current EN 590 diesel fuel specification.

A fuel composition prepared according to the present invention may be for example an automotive gasoline or diesel fuel composition, in particular the latter.

A gasoline fuel composition prepared according to the present invention may in general be any type of gasoline fuel composition suitable for use in a spark ignition (petrol) engine. It may contain, in addition to the VI improving additive, other standard gasoline fuel components. It may, for example, include a major proportion of a gasoline base fuel, which will typically have a boiling range (ASTM D-86 or EN ISO 3405) of from 20 to 210° C. A “major proportion” in this context means typically 85% w/w or greater based on the overall fuel composition, more suitably 90 or 95% w/w or greater, most preferably 98 or 99 or 99.5% w/w or greater.

A diesel fuel composition prepared according to the present invention may in general be any type of diesel fuel composition suitable for use in a compression ignition (diesel) engine. It may contain, in addition to the VI improving additive, other standard diesel fuel components. It may, for example, include a major proportion of a diesel base fuel, for instance of the type described below. Again a “major proportion” means typically 85% w/w or greater based on the overall composition, more suitably 90 or 95% w/w or greater, most preferably 98 or 99 or 99.5% w/w or greater.

Thus, in addition to the VI improving additive, a diesel fuel composition prepared according to the present invention may comprise one or more diesel fuel components of conventional type. Such components will typically comprise liquid hydrocarbon middle distillate fuel oil(s), for instance petroleum derived gas oils. In general such fuel components may be organically or synthetically derived, and are suitably obtained by distillation of a desired range of fractions from a crude oil. They will typically have boiling points within the usual diesel range of 150 to 410° C. or 170 to 370° C., depending on grade and use. Typically the fuel composition will include one or more cracked products, obtained by splitting heavy hydrocarbons.

A petroleum derived gas oil may for instance be obtained by refining and optionally (hydro)processing a crude petroleum source. It may be a single gas oil stream obtained from such a refinery process or a blend of several gas oil fractions obtained in the refinery process via different processing routes. Examples of such gas oil fractions are straight run gas oil, vacuum gas oil, gas oil as obtained in a thermal cracking process, light and heavy cycle oils as obtained in a fluid catalytic cracking unit and gas oil as obtained from a hydrocracker unit. Optionally a petroleum derived gas oil may comprise some petroleum derived kerosene fraction.

Such gas oils may be processed in a hydrodesulphurisation (HDS) unit so as to reduce their sulphur content to a level suitable for inclusion in a diesel fuel composition.

A diesel base fuel may be or comprise a Fischer-Tropsch derived diesel fuel component, typically a Fischer-Tropsch derived gas oil. In the context of the present invention, the term “Fischer-Tropsch derived” means that a material is, or derives from, a synthesis product of a Fischer-Tropsch condensation process. The term “non-Fischer-Tropsch derived” may be interpreted accordingly. A Fischer-Tropsch derived fuel or fuel component will therefore be a hydrocarbon stream in which a substantial portion, except for added hydrogen, is derived directly or indirectly from a Fischer-Tropsch condensation process.

The Fischer-Tropsch reaction converts carbon monoxide and hydrogen into longer chain, usually paraffinic, hydrocarbons:

n(CO+2H₂)═(—CH₂-)_(n) +nH₂O+heat,

in the presence of an appropriate catalyst and typically at elevated temperatures (e.g. 125 to 300° C., preferably 175 to 250° C.) and/or pressures (e.g. 0.5 to 10 MPa, preferably 1.2 to 5 MPa). Hydrogen:carbon monoxide ratios other than 2:1 may be employed if desired.

The carbon monoxide and hydrogen may themselves be derived from organic, inorganic, natural or synthetic sources, typically either from natural gas or from organically derived methane.

A Fischer-Tropsch derived diesel fuel component of use in the present invention may be obtained directly from the refining or the Fischer-Tropsch reaction, or indirectly for instance by fractionation or hydrotreating of the refining or synthesis product to give a fractionated or hydrotreated product. Hydrotreatment can involve hydrocracking to adjust the boiling range (see e.g. GB-B-2077289 and EP-A-0147873) and/or hydroisomerisation which can improve cold flow properties by increasing the proportion of branched paraffins. EP-A-0583836 describes a two-step hydrotreatment process in which a Fischer-Tropsch synthesis product is firstly subjected to hydroconversion under conditions such that it undergoes substantially no isomerisation or hydrocracking (this hydrogenates the olefinic and oxygen-containing components), and then at least part of the resultant product is hydroconverted under conditions such that hydrocracking and isomerisation occur to yield a substantially paraffinic hydrocarbon fuel. The desired fraction(s), typically gas oil fraction(s), may subsequently be isolated for instance by distillation.

Other post-synthesis treatments, such as polymerisation, alkylation, distillation, cracking-decarboxylation, isomerisation and hydroreforming, may be employed to modify the properties of Fischer-Tropsch condensation products, as described for instance in U.S. Pat. Nos. 4,125,566 and 4,478,955.

Typical catalysts for the Fischer-Tropsch synthesis of paraffinic hydrocarbons comprise, as the catalytically active component, a metal from Group VIII of the periodic table of the elements, in particular ruthenium, iron, cobalt or nickel. Suitable such catalysts are described for instance in EP-A-0583836.

An example of a Fischer-Tropsch based process is the Shell™ “Gas-to-liquids” or “GtL” technology (formerly known as the SMDS (Shell Middle Distillate Synthesis) and described in “The Shell Middle Distillate Synthesis Process”, van der Burgt et al, paper delivered at the 5th Synfuels Worldwide Symposium, Washington D.C., November 1985, and in the November 1989 publication of the same title from Shell International Petroleum Company Ltd, London, UK). In the latter case, preferred features of the hydroconversion process may be as disclosed therein. This process produces middle distillate range products by conversion of a natural gas into a heavy long chain hydrocarbon (paraffin) wax which can then be hydroconverted and fractionated.

For use in the present invention, a Fischer-Tropsch derived fuel component is preferably any suitable component derived from a gas to liquid synthesis (hereinafter a GtL component), or a component derived from an analogous Fischer-Tropsch synthesis, for instance converting gas, biomass or coal to liquid (hereinafter an XtL component). A Fischer-Tropsch derived component is preferably a GtL component. It may be a BtL (biomass to liquid) component. In general a suitable XtL component may be a middle distillate fuel component, for instance selected from kerosene, diesel and gas oil fractions as known in the art; such components may be generically classed as synthetic process fuels or synthetic process oils. Preferably an XtL component for use as a diesel fuel component is a gas oil.

Diesel fuel components contained in a composition prepared according to the present invention will typically have a density of from 750 to 900 kg/m³, preferably from 800 to 860 kg/m³, at 15° C. (ASTM D-4052 or EN ISO 3675) and/or a VK 40 of from 1.5 to 6.0 mm²/s (ASTM D-445 or EN ISO 3104).

In a diesel fuel composition prepared according to the present invention, the base fuel may itself comprise a mixture of two or more diesel fuel components of the types described above. It may be or contain a so-called “biodiesel” fuel component such as a vegetable oil, hydrogenated vegetable oil or vegetable oil derivative (e.g. a fatty acid ester, in particular a fatty acid methyl ester) or another oxygenate such as an acid, ketone or ester. Such components need not necessarily be bio-derived.

An automotive diesel fuel composition prepared according to the present invention will suitably comply with applicable current standard specification(s) such as for example EN 590 (for Europe) or ASTM D-975 (for the USA). By way of example, the overall fuel composition may have a density from 820 to 845 kg/m³ at 15° C. (ASTM D-4052 or EN ISO 3675); a T95 boiling point (ASTM D-86 or EN ISO 3405) of 360° C. or less; a measured cetane number (ASTM D-613) of 51 or greater; a VK 40 (ASTM D-445 or EN ISO 3104) from 2 to 4.5 mm²/s; a sulphur content (ASTM D-2622 or EN ISO 20846) of 50 mg/kg or less; and/or a polycyclic aromatic hydrocarbons (PAH) content (IP 391(mod)) of less than 11% w/w. Relevant specifications may, however, differ from country to country and from year to year, and may depend on the intended use of the fuel composition.

A diesel fuel composition prepared according to the present invention suitably contains no more than 5000 ppmw (parts per million by weight) of sulphur, typically from 2000 to 5000 ppmw, or from 1000 to 2000 ppmw, or alternatively up to 1000 ppmw. The composition may, for example, be a low or ultra low sulphur fuel, or a sulphur free fuel, for instance containing at most 500 ppmw, preferably no more than 350 ppmw, most preferably no more than 100 or 50 or even 10 ppmw, of sulphur.

An automotive fuel composition prepared according to the present invention, or a base fuel used in such a composition, may be additivated (additive-containing) or unadditivated (additive-free). If additivated, e.g. at the refinery, it will contain minor amounts of one or more additives selected for example from anti-static agents, pipeline drag reducers, flow improvers (e.g. ethylene/vinyl acetate copolymers or acrylate/maleic anhydride copolymers), lubricity additives (other than the VI improving additive described hereinabove), antioxidants and wax anti-settling agents. Thus, the composition may contain a minor proportion (preferably 1% w/w or less, more preferably 0.5% w/w (5000 ppmw) or less and most preferably 0.2% w/w (2000 ppmw) or less), of one or more fuel additives, in addition to the VI improving additive.

The composition may for example contain a detergent. Detergent-containing diesel fuel additives are known and commercially available. Such additives may be added to diesel fuels at levels intended to reduce, remove or slow the build up of engine deposits.

Examples of detergents suitable for use in fuel additives for the present purpose include polyolefin substituted succinimides or succinamides of polyamines, for instance polyisobutylene succinimides or polyisobutylene amine succinamides, aliphatic amines, Mannich bases or amines and polyolefin (e.g. polyisobutylene) maleic anhydrides. Succinimide dispersant additives are described for example in GB-A-960493, EP-A-0147240, EP-A-0482253, EP-A-0613938, EP-A-0557516 and WO-A-98/42808. Particularly preferred are polyolefin substituted succinimides such as polyisobutylene succinimides.

A fuel additive mixture useable in a fuel composition prepared according to the present invention may contain other components in addition to the detergent. Examples are lubricity enhancers; dehazers, e.g. alkoxylated phenol formaldehyde polymers; anti-foaming agents (e.g. polyether-modified polysiloxanes); ignition improvers (cetane improvers) (e.g. 2-ethylhexyl nitrate (EHN), cyclohexyl nitrate, di-tert-butyl peroxide and those disclosed in U.S. Pat. No. 4,208,190 at column 2, line 27 to column 3, line 21); anti-rust agents (e.g. a propane-1,2-diol semi-ester of tetrapropenyl succinic acid, or polyhydric alcohol esters of a succinic acid derivative, the succinic acid derivative having on at least one of its alpha-carbon atoms an unsubstituted or substituted aliphatic hydrocarbon group containing from 20 to 500 carbon atoms, e.g. the pentaerythritol diester of polyisobutylene-substituted succinic acid); corrosion inhibitors; reodorants; anti-wear additives; antioxidants (e.g. phenolics such as 2,6-di-tert-butylphenol, or phenylenediamines such as N,N′-di-sec-butyl-p-phenylenediamine); metal deactivators; combustion improvers; static dissipator additives; cold flow improvers; and wax anti-settling agents.

Such a fuel additive mixture may contain a lubricity enhancer (in addition to the viscosity improving (VI) additive described hereinabove), especially when the fuel composition has a low (e.g. 500 ppmw or less) sulphur content. In the additivated fuel composition, the lubricity enhancer is conveniently present at a concentration of less than 1000 ppmw, preferably between 50 and 1000 ppmw, more preferably between 70 and 1000 ppmw. Suitable commercially available lubricity enhancers include ester- and acid-based additives. Other lubricity enhancers are described in the patent literature, in particular in connection with their use in low sulphur content diesel fuels, for example in:

-   -   the paper by Danping Wei and H. A. Spikes, “The Lubricity of         Diesel Fuels”, Wear, III (1986) 217-235;     -   WO-A-95/33805—cold flow improvers to enhance lubricity of low         sulphur fuels;     -   WO-A-94/17160—certain esters of a carboxylic acid and an alcohol         wherein the acid has from 2 to 50 carbon atoms and the alcohol         has 1 or more carbon atoms, particularly glycerol monooleate and         di-isodecyl adipate, as fuel additives for wear reduction in a         diesel engine injection system;     -   U.S. Pat. No. 5,490,864—certain dithiophosphoric         diester-dialcohols as anti-wear lubricity additives for low         sulphur diesel fuels; and     -   WO-A-98/01516—certain alkyl aromatic compounds having at least         one carboxyl group attached to their aromatic nuclei, to confer         anti-wear lubricity effects particularly in low sulphur diesel         fuels.

Since inclusion of the viscosity improving (VI) additives disclosed hereinabove can improve the lubricity of the fuel composition, an advantage of the present invention is that the amount of other lubricity additives can be reduced or even eliminated.

It may also be preferred for the fuel composition to contain an anti-foaming agent, more preferably in combination with an anti-rust agent and/or a corrosion inhibitor and/or a lubricity enhancing additive.

Unless otherwise stated, the (active matter) concentration of each such additive component in the additivated fuel composition is preferably up to 10000 ppmw, more preferably in the range of 0.1 to 1000 ppmw, advantageously from 0.1 to 300 ppmw, such as from 0.1 to 150 ppmw.

The (active matter) concentration of any dehazer in the fuel composition will preferably be in the range from 0.1 to 20 ppmw, more preferably from 1 to 15 ppmw, still more preferably from 1 to 10 ppmw, advantageously from 1 to 5 ppmw. The (active matter) concentration of any ignition improver present will preferably be 2600 ppmw or less, more preferably 2000 ppmw or less, conveniently from 300 to 1500 ppmw. The (active matter) concentration of any detergent in the fuel composition will preferably be in the range from 5 to 1500 ppmw, more preferably from 10 to 750 ppmw, most preferably from 20 to 500 ppmw.

If desired, one or more additive components, such as those listed above, may be co-mixed—preferably together with suitable diluent(s)—in an additive concentrate, and the additive concentrate may then be dispersed into a base fuel or fuel composition. The VI improving additive may, in accordance with the present invention, be incorporated into such an additive formulation.

In the case of a diesel fuel composition, for example, the fuel additive mixture will typically contain a detergent, optionally together with other components as described above, and a diesel fuel-compatible diluent, which may be a mineral oil, a solvent such as those sold by Shell companies under the trade mark “SHELLSOL”, a polar solvent such as an ester and, in particular, an alcohol, e.g. hexanol, 2-ethylhexanol, decanol, isotridecanol and alcohol mixtures such as those sold by Shell companies under the trade mark “LINEVOL”, especially LINEVOL 79 alcohol which is a mixture of C₇₋₉ primary alcohols, or a C₁₂₋₁₄ alcohol mixture which is commercially available.

The total content of the additives in the fuel composition may be suitably between 0 and 10000 ppmw and preferably below 5000 ppmw.

In this specification, amounts (concentrations, % v/v, ppmw, % w/w) of components are of active matter, i.e. exclusive of volatile solvents/diluent materials.

Different types and/or concentrations of additives may be appropriate for use in gasoline fuel compositions, which for example may contain polyisobutylene/amine and/or polyisobutylene/amide copolymers as detergent additives.

Suitably, the VI improving additive, and the concentration at which it is used in the fuel composition, will be such as to cause an increase in the cold filter plugging point (CFPP) of the composition of 10° C. or less, preferably 5 or 2 or 1° C. or less. Preferably it will be such as to cause no increase in CFPP. In cases it may be such as to cause a decrease in CFPP. Increases in CFPP may be as compared to the CFPP of the fuel composition prior to incorporation of the VI improving additive. They may be as compared to the CFPP of an otherwise analogous fuel composition which is intended (e.g. marketed) for use in an internal combustion (in particular diesel) engine, prior to adding a VI improving additive to it. CFPPs may be measured using the standard test method EN 116.

Suitably, the VI improving additive, and the concentration at which it is used in the fuel composition, will be such as to cause an increase in the cloud point of the composition of 10° C. or less, preferably 5 or 2 or 1° C. or less. Preferably it will be such as to cause no increase in cloud point. In cases it may be such as to cause a decrease in cloud point. Increases in cloud point may be as compared to that of the fuel composition prior to incorporation of the VI improving additive. They may be as compared to the cloud point of an otherwise analogous fuel composition which is intended (e.g. marketed) for use in an internal combustion (in particular diesel) engine, prior to adding a VI improving additive to it. Cloud points may be measured using the standard test method EN 23015.

In the context of the present invention, “use” of a VI improving additive in a fuel composition means incorporating the VI improving additive into the composition, typically as a blend (i.e. a physical mixture) with one or more fuel components (typically diesel base fuels) and optionally with one or more fuel additives. The VI improving additive is conveniently incorporated before the composition is introduced into an engine which is to be run on the composition. Instead or in addition the use may involve running an engine on the fuel composition containing the VI improving additive, typically by introducing the composition into a combustion chamber of the engine.

“Use” of a VI improving additive, in accordance with the present invention, may also embrace supplying such an additive together with instructions for its use in an automotive fuel composition to achieve one or more of the purpose(s) described above, in particular to improve the acceleration performance of an internal combustion (typically diesel) engine into which the composition is, or is intended to be, introduced.

The VI improving additive may itself be supplied as a component of a formulation which is suitable for and/or intended for use as a fuel additive, in particular a diesel fuel additive, in which case the VI improving additive may be included in such a formulation for the purpose of influencing its effects on the lubricity of an automotive fuel composition, and/or its effects on the acceleration performance and/or power output of an engine into which a fuel composition is, or is intended to be, introduced.

Thus, the VI improving additive may be incorporated into an additive formulation or package along with one or more other fuel additives. It may, for instance, be combined, in an additive formulation, with one or more fuel additives selected from detergents, anti-corrosion additives, esters, poly alpha olefins, long chain organic acids, components containing amine or amide active centres, and mixtures thereof. In particular, it may be combined with one or more so-called performance additives, which will typically include at least a detergent.

The VI improving additive may be dosed directly into a fuel component or composition, for example at the refinery. It may be pre-diluted in a suitable fuel component which subsequently forms part of the overall automotive fuel composition.

In accordance with the present invention, two or more VI improving additives may be used in an automotive fuel composition for the purpose(s) described above.

According to a further aspect of the present invention, there is provided a process for the preparation of an automotive fuel composition, which process involves blending an automotive base fuel with a VI improving additive, wherein the VI improving additive is a star-shaped isoprene polymer. The blending may be carried out for one or more of the purposes described above, in particular with respect to the lubricity of the resultant fuel composition and/or its effect on the acceleration performance and/or power output of an internal combustion engine into which it is, or is intended to be, introduced. The composition may in particular be a diesel fuel composition.

The VI improving additive may, for example, be blended with other components of the composition, in particular the base fuel, at the refinery. Alternatively, it may be added to an automotive fuel composition downstream of the refinery. It may be added as part of an additive package which contains one or more other fuel additives.

A further aspect of the present invention provides a method of operating an internal combustion engine, and/or a vehicle which is powered by such an engine, which method involves introducing into a combustion chamber of the engine a fuel composition described hereinabove. Again the fuel composition is preferably introduced for one or more of the purposes described in connection with the present invention. Thus, the engine is preferably operated with the fuel composition for the purpose of improving its lubricity and/or acceleration performance and/or power output.

Again the engine may in particular be a diesel engine. It may be a turbo charged engine, in particular a turbo charged diesel engine. The diesel engine may be of the direct injection type, for example of the rotary pump, in-line pump, unit pump, electronic unit injector or common rail type, or of the indirect injection type. It may be a heavy or a light duty diesel engine. It may in particular be an electronic unit direct injection (EUDI) engine.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, mean “including but not limited to”, and do not exclude other moieties, additives, components, integers or steps.

Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise.

Preferred features of each aspect of the present invention may be as described in connection with any of the other aspects.

Other features of the present invention will become apparent from the following examples. Generally speaking, the present invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims and drawings). Thus features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the present invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith.

Moreover, unless stated otherwise, any feature disclosed herein may be replaced by an alternative feature serving the same or a similar purpose.

The following examples illustrate the properties of automotive fuel compositions prepared according to the present invention, and assess the effects of such compositions on the performance of a diesel engine.

EXAMPLES

Fuel blends were prepared by combining a diesel base fuel (meeting the European diesel fuel specification EN590) with a viscosity index (VI) improving additive. The viscosity index (VI) additives used in the present experiments were SV150, SV260, SV300 and SV600, at a treat rate of either 500 mg/kg or 1000 mg/kg.

SV150 is a linear, di-block polymer commercially available from Infineum and is used in the present examples as a comparison.

SV260 is a star-shaped styrene-polyisoprene polymer commercially available from Infineum.

SV300 is a star-shaped styrene-polyisoprene polymer commercially available from Infineum.

SV600 is a star-shaped styrene-polyisoprene polymer commercially available from Infineum.

Before being added to the diesel base fuel, the VI improving additives were pre-blended with Shellsol A150 solvent (commercially available from Shell). The weight ratio of VI improving additive to Shellsol A150 was 1:8.

The fuel specification of the diesel base fuel used in the present examples is shown in Table 1 below.

TABLE 1 Specification of Diesel Base Fuel (CEC RF-79-07) Limits Property Units Min. Max. Cetane Number — 52.0 54.0 Density at 15° C. kg/m³ 833.0 837.0 Distillation IBP ° C. — — Distillation 10% v/v ° C. — — Distillation 50% v/v ° C. 245.0 — Distillation 90% v/v ° C. — — Distillation 95% v/v ° C. 345.0 350.0 Distillation FBP ° C. — 370.0 Flash Point ° C. 62 — CFPP ° C. — −5 Viscosity at 40° C. mm²/ s 2300 3300 Aromatics, Total % wt — — Aromatics, Mono % wt — — Aromatics, Di Wt % — — Aromatics, Tri+ Wt % — — Aromatics, Poly % wt 3.0 6.0 (2 + 3) Sulfur mg/kg — 10 Corrosion - Copper — Max 1 Carbon Residue on % wt 0.2 10% Distillation Residue Ash Content % wt — 0.010 Water % wt — 0.0200 Strong Acid Number mg/KOH/g — 0.02 Oxidation Stability mg/mL — 0.025 Carbon % wt — — Hydrogen % wt — — C:H Ratio (H = 1) — — H:C Ratio (C = 1) — — Net Heating Value MJ/kg — — Net Heating Value Btu/lb — — HFRR (wsd 1, 4) μm — 400 Element Analysis — Al mg/kg — 0.1 Ag mg/kg — 0.1 B mg/kg — 0.1 Ba mg/kg — 0.1 Ca mg/kg — 0.1 Cd mg/kg — 0.1 Ce mg/kg — 0.1 Cr mg/kg — 0.1 Cu mg/kg — 0.1 Fe mg/kg — 0.1 K mg/kg — 0.1 Mg mg/kg — 0.1 Sn mg/kg — 0.1 Mn mg/kg — 0.1 Mo mg/kg — 0.1 Na mg/kg — 0.1 Ni mg/kg — 0.1 P mg/kg — 0.1 Pb mg/kg — 0.1 Si mg/kg — 0.1 Sn mg/kg — 0.1 Ti mg/kg — 0.1 V mg/kg — 0.1 Ni mg/kg — 0.1 Zn mg/kg — 0.1

Demonstration of Power Benefit

The fuel blends described above, containing a VI additive pre-blended in Shellsol A150, were used in a bench test engine in order to assess the effects of the VI improving additive on the power performance of an engine. The bench test engine chosen for this study was a PSA DW10B. This engine is specified in CEC F-98-09, the industry standard test for injector nozzle fouling in modern DI car engines, and as such there is a vast amount of historical test data available supporting the performance and characteristics of the engine. DW10B test details are listed in Table 2 below:

TABLE 2 Manufacturer Peugeot Engine Code DW10XE Displacement (ltr)/layout 2.0/In line 4 Max Power (kW) @ (rpm) 100 kW@4000 r/min Max Torque (Nm) (rpm) 320 Nm@2000 r/min Injection Type/Manufacturer Common Rail (CR)/Continental, 1600 bar EMS Manufacturer Continental Emissions Class Euro 4 Lubricant Shell Helix Ultra

A constant test speed of 4000 r/min was used and maximum accelerator pedal position (100% APP) was applied using DW10B CEC specification nozzles. The engine was run in cycles of 24 minutes per fuel, alternating between base fuel without additive and candidate fuel, and the power output was measured. The results of the power testing are set out in Table 3 below. The power benefit (%) is compared to the unadditised base fuel (containing Shellsol A150) (designated as Example 1 in Table 3).

Demonstration of Lubricity

The fuel blends were also subjected to an HFRR test (according to ISO 12156) in order to measure their lubricity. The HFRR (High Friction Reciprocating Rig) is a controlled reciprocating friction and wear testing device employed to assess the lubricity performance of fuels and lubricants. The test uses a 6 mm diameter steel ball loaded and reciprocated against the flat surface of a stationary steel disc immersed in fuel. At the end of each test, the ball and disc are removed from the test rig, rinsed with toluene and iso-propanol, and then treated with a 0.05 wt % solution of ethylenediaminetetraacetic acid (EDTA) for 60 s. Topography images were then obtained and analysed to determine wear volumes of the wear scars on the ball and the disc using the SWLI Veeco Wyko model NT9100. The instrument was set in Vertical Scanning Interferometry (VSI) mode, calibrated to measure rough surfaces with a nanometer detection range. The results of the HFRR tests are set out in Table 4 below. The % change in wear scar diameter is compared to the unadditised base fuel (containing Shellsol A150) (designated in Table 4 as Example 8). A negative result in % change denotes a benefit.

TABLE 3 ppm of Shellsol Dose of VII Power Benefit E.g. A150 VII (mg/kg) (%)  1* 8000 — — — 2 8000 SV150 1000 0.46 3 4000 SV150 500 0.20 4 8000 SV300 1000 0.67 5 4000 SV300 500 0.35 6 8000 SV600 1000 0.36 7 4000 SV600 500 0.09 ppm of Wear scar Delta wear scar Lubricity Shellsol Dose of VII diameter diameter (% change in wear E.g. A150 VII (mg/kg) (μm) (μm) scar diameter)  8* 8000 — — 359 — —  9 8000 SV150 1000 339 −20 −5.6 10 4000 SV150 500 346 −13 −3.6 11 8000 SV300 1000 349 −10 −2.8 12 4000 SV300 500 320 −39 −10.9 13 8000 SV600 1000 192 −167 −46.5 14 4000 SV600 500 331 −28 −7.8 15 8000 SV260 1000 298 −61 −17.0 *Not according to the present invention

DISCUSSION

The fuel blend which contained the star-shaped styrene-polyisoprene polymer SV300 showed a significant increase in power benefit (both at 1000 mg/kg and 500 mg/kg) (see Examples 4 and 5) compared with the fuel blends which contained the linear, di-block polymer SV150 (see Examples 2 and 3).

The fuel blend which contained the star-shaped styrene-polyisoprene polymer SV600 at 1000 mg/kg showed an increase in power benefit compared to the base fuel (see Examples 6 and 7). Although the fuel blend containing the star-shaped styrene-polyisoprene polymer SV600 at 1000 mg/kg did not show as large a power benefit as the fuel blend containing the linear, di-block polymer SV150, it exhibited significantly better lubricity performance (see Examples 6 and 13).

The fuel blend which contained 1000 mg/kg of the star-shaped styrene-polyisoprene polymer SV260 (Example 15) showed improved lubricity performance compared to the fuel blend containing 1000 mg/kg of SV150 (Example 9). 

1. A fuel composition comprising a base fuel and at least one viscosity index (VI) improving additive, wherein the viscosity index (VI) improving additive is a star-shaped polyisoprene-based polymer.
 2. The fuel composition according to claim 1 wherein the star-shaped polyisoprene-based polymer is characterized by the formula: (D′-PA-D″)n-X; wherein D′ represents a block derived from at least one diene; PA represents a block derived from monoalkenyl arene; D″ represents a block derived from diene; n represents the average number of arms per star polymer formed by the reaction of 2 or more moles of a polyalkenyl coupling agent per mole of arms; and X represents a nucleus of a polyalkenyl coupling agent; wherein at least one of diene blocks D′ and D″ is a copolymer block derived from mixed diene monomer, in which from about 65 wt % to about 95 wt % of the incorporated monomer units are from isoprene and from about 5 wt % up to about 35 wt % of the incorporated monomer units are from butadiene, and wherein at least about 80 wt % of the butadiene is incorporated in a 1,4-configuration; and wherein D′ has a number average molecular weight of from about 10,000 to about 120,000 daltons; PA has a number average molecular weight of from about 10,000 to about 50,000 daltons; and D″ has a number average molecular weight of from about 5,000 to about 60,000 daltons.
 3. The fuel composition according to claim 1 wherein the star-shaped polyisoprene-based polymer comprises a crosslinked polystyrene core with arms of hydrogenated polyisoprene or poly(alternated ethylene-propylene).
 4. The fuel composition according to claim 1 wherein the fuel composition is a diesel fuel composition.
 5. The fuel composition according to claim 1 wherein the concentration of the VI improving additive in the fuel composition is from 0.001 to 0.5% w/w, preferably from 0.05 to 0.25% w/w.
 6. A use of a viscosity index (VI) improving additive in an automotive fuel composition, for the purpose of improving the lubricity of the fuel composition, wherein the viscosity index (VI) improving additive is a star-shaped polyisoprene-based polymer.
 7. A use of a viscosity index (VI) improving additive in an automotive fuel composition, for the purpose of improving the power output of an internal combustion engine into which the viscosity index (VI) improving additive, or an automotive fuel composition containing the viscosity index (VI) improving additive, is or is intended to be introduced or of a vehicle powered by such an engine, wherein the viscosity index (VI) improving additive is a star-shaped polyisoprene-based polymer.
 8. A use of a viscosity index (VI) improving additive in an automotive fuel composition, for the purpose of improving the lubricity of the fuel composition at the same time as improving the power output of an internal combustion engine into which the viscosity index (VI) improving additive, or an automotive fuel composition containing the viscosity index (VI) improving additive, is or is intended to be introduced or of a vehicle powered by such an engine, wherein the viscosity index (VI) improving additive is a star-shaped polyisoprene-based polymer.
 9. The use according to claim 7, wherein the VI improving additive is pre-dissolved in a solvent or fuel component.
 10. (canceled)
 11. The use according to claim 8, wherein the VI improving additive is pre-dissolved in a solvent or fuel component. 